As a method for producing an optical element formed of a glass by press-molding without grinding or polishing, a precision press-molding is known. This method is a method in which optical elements that are not easily produced by grinding and polishing such as aspherical lenses, microlenses, diffraction gratings, etc., are highly productively mass-produced by accurately transferring the form of inner surface of a press mold to each glass material that is being molded.
The glass material to be molded by the precision press-molding method is called a preform, and it is a glass shaped material formed of a glass having a weight accurately equivalent to the weight of a press-molded product. When the steps from the melting of a glass up to the formation of an optical element are viewed as a series of process, the productivity of the process as a whole can be more improved if a preform can be shaped directly from a molten glass.
Meanwhile, a fluorine-containing glass such as fluorophosphate glass or the like is a very useful optical glass as a low-dispersion glass, and as this fluorophosphate optical glass, a glass as described in Japanese National Publication of Translated Version No. 3-500162 is known.